1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for transmitting an RPC (Reverse Power Control) channel in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
Long signal paths and shadowing worsen signal attenuation, inter-system interference, and fading in a radio environment. Thus the carrier-to-interference ratio (C/I) of a signal varies greatly according to channel condition. Standardized mobile communication systems adopt link adaptive techniques of controlling a data rate according to channel condition or C/I to increase channel throughput. The data rate is determined by a code rate and a modulation scheme. When the C/I is higher, the data rate is increased by using a higher code rate and a higher-order modulation. When the C/I is lower, the data rate is decreased by using a lower code rate and a lower-order modulation, thereby increasing channel reliability. A receiver estimates channel condition on the basis of a C/I, determines a data rate according to the estimated channel condition, and feeds back information about the data rate to a transmitter. Then the transmitter assigns the requested data rate to the receiver.
The Third Generation Partnership Project 2 (3GPP2) has established 1× EV-DO (Evolution-Data Only) and HDR (High Data Rate) standards for the purpose of supporting high-speed data service based on cdma200 1× standards. According to the standards, a transmitter is known as an AN (Access Network) and a receiver is known as an AT (Access Terminal) if it is forward link. On the 1× EV-DO physical layer adopting a link adaptive technique, 12 data rates are available according to three modulation schemes including QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), and 16QAM (16-ary Quadrature Amplitude Modulation), two code rates (i.e., ⅕ and ⅓), and packet length.
An AT determines a data rate at which it can receive a forward traffic channel by measuring the C/I of a forward pilot channel and feeds back data rate information to an AN so that the AN can select a forward data rate based on the feed-back information. The data rate information is DRC (Data Rate Control) information. The DRC information is represented by a 4-bit DRC symbol transmitted on a DRC channel. Aside from the DRC information, the AT transmits 3-bit information indicating a sector that will be serviced among eight effective sectors on the DRC channel. The 3-bit sector information is defined as a DRC cover indicative of the index of an orthogonal code for covering such as a Walsh code.
FIG. 1 is a block diagram of channel structure in an AN of a conventional mobile communication system supporting high-speed data transmission. The channel structure is comprised of a traffic and control channel 101 to 108, a preamble 111 and 112, a MAC (Media Access Control) channel 121 to 123 and 131 to 134, and a pilot channel 141.
In the traffic and control channel, an encoder 101 encodes forward traffic/control channel information. A scrambler 102 scrambles the code symbols received from the encoder 101 with a predetermined scrambling code and a modulator 103 modulates the scrambled symbols by one of QPSK, 8PSK and 16QAM according to a data rate. A puncturing and repetition unit 104 punctures and repeats the modulation symbols received from the modulator 103 in a predetermined rule to match the data rate. A symbol demultiplexer (DEMUX) 105 demultiplexes the output of the puncturing and repetition unit 104. The outputs of the symbol DEMUX 105 are spread with orthogonal codes such as Walsh codes. A channel gain processor 107 multiplies each spread channel signal by a predetermined gain (e.g., ¼) and a chip level summer 108 sums the outputs of the channel gain processor 107 on a chip level.
In the preamble, a spreader 111 spreads a preamble signal with a Walsh code Wi32 assigned according to a MAC index and a preamble repeater 112 repeats the spread signal a predetermined number of times according to the data rate.
A MAC channel is divided into an RA (Reverse Activity) channel and an RPC channel. Each channel is spread with a length 64 Walsh code. An RA bit repeater 121 repeats a 1-bit RAB (Reverse Activity Bit) according to a repetition factor set in RABLength. The RAB provides information about reverse link interference load and is broadcast to all ATs within the sector. An RA channel gain processor 122 multiplies the output of the RA bit repeater 121 by an RA channel gain and a spreader 123 spreads the output of the RA gain processor 122 with a predetermined Walsh code W464. An RPC channel gain processor 131 multiplies RPC bits by a channel gain G(i). The RPC bits indicate reverse power control information for an AT with MACIndex i. The AN measures the C/I of the reverse link from the AT with MACIndex i. If the C/I is lower than a threshold, the RPC bits are set to ‘0’ (UP) and if the C/I is higher than the threshold, they are set to ‘1’ (DOWN). A spreader 132 spreads the output of the RPC channel gain processor 131 with a predetermined Walsh code Wi64. A chip level summer 133 sums the outputs of the spreaders 123 and 132 at a chip level. A repeater 134 repeats the sum according to a predetermined repetition factor (e.g., 4). The transmission power of the RA channel and the RPC channel is maintained to be the same as that of the traffic, control, and pilot channels.
In the pilot channel, a spreader 141 spreads a pilot signal with all 0s on an In-phase channel with a predetermined Walsh code, Walsh 0.
A time-division multiplexer (MUX) 151 time-division multiplexes the outputs of the traffic and control channel, the preamble, the MAC channel, and the pilot channel according to a predetermined rule. A complex spreader 152 complex-spreads the outputs of the time-division MUX 151 with a predetermined PN (Pseudo Noise) code. A baseband filter 153 baseband-filters the PN-spread signal. The resulting signal is modulated with a corresponding carrier and transmitted to an AT through an antenna. Here, the transmission power is maintained to be reference transmission power, which can be set usually to the highest transmission power level of the transmitter.
FIGS. 2A and 2B illustrate the structures of slots in which the multiplexed forward traffic/control channel, MAC channel, and pilot channel output from the time-division MUX 151. FIG. 2A illustrates an active slot in which a traffic/control channel is delivered. In the active slot, each of two 96-chip pilot bursts is located at the center of each half slot. A 64-chip symbol of the MAC channel containing the RA channel and the RPC channel occurs four times in the slot, before and after the two pilot bursts. The remaining 1600 chips of the slot are occupied by the traffic/control channel.
FIG. 2B illustrates an idle slot free of the traffic/control channel. In the idle slot, only the pilot channel and the MAC channel are delivered. The AN transmits the time-division multiplexed traffic/control, MAC, and pilot channels with its highest transmission power on the forward link.
The MAC channel includes one RA channel and up to 59 RPC channels that are code-division multiplexed using 64 Walsh codes. Each of the channels is transmitted with its channel gain. A MAC index is assigned to the RA channel and each RPC channel. Each channel is spread with a 64 Walsh code corresponding to its assigned MAC index. For example, Walsh code 4 is assigned to the RA channel and different Walsh codes between Walsh code 5 to Walsh code 63 are assigned to each RPC channel. Since the RA channel is broadcast to all ATs within a sector, its channel gain is determined such that the reception energy of RA channels accumulated in as many slots as RABLength with respect to an AT at a cell boundary satisfies a reference RF error performance. On the other hand, each RPC channel is destined for a specific AT within the sector and up to 59 RPC channels exist. Because of limited power available to all the RPC channels, a channel gain sufficient to satisfy a reference RPC error performance cannot be assigned to each AT within the sector. As the number of ATs increases or an AT moves farther from an AN, channel gain requirements are increased. If the sum of RPC channel gains required to ensure sufficient RPC performance for all ATs is higher than the total power assigned to all of the RPC channels, a required channel gain cannot be assigned to each RPC channel or an RPC channel cannot be assigned to some ATs. As a result, the power control performance of each AT is seriously deteriorated. As the number of ATs increases, as more ATs are located at a cell boundary, and as the reception channel condition of ATs are bad, an RPC channel power assigned to each AT becomes less.
Some of ATs within the sector may be in a soft handoff situation and thus are assigned RPC channels from at least two sectors.
If received RPC bits are identical, the AT detects the RPC bit by diversity-combining the RPC channels received from the sectors. On the other hand, if the RPC bits are different, the AT detects them and increases its transmission power only when both RPC bits are ‘0’ (UP). That is, an RPC channel delivers information for controlling the transmission power of an AT.
As described above, one AT occupies RPC channels from at least two sectors to achieve the diversity gain of the RPC channels at a soft handoff. However, power that could be assigned to an RPC channel for another AT is consumed for the benefit of the diversity gain, which causes a potential RPC channel power shortage and decreases the power control performance of the AT. What is worse, since the AT at soft-handoff is near a cell boundary and experiences degraded channel condition, it requires a higher channel gain to satisfy corresponding RPC channel error performance. If there are many concurrent ATs at soft-handoff, the RPC channel power shortage becomes more severe. Although the number of ATs at soft-handoff varies with sector size, AT distribution within the sector, and channel condition, a soft-handoff AT consumes much more RPC channel power than an AT in a normal operating situation.
Thus it can be concluded that since at soft handoff, an AT requires relatively high RPC channel power from at least two sectors, leading to a potential RPC channel power shortage, degrading the overall reverse link power control performance and simultaneously reducing the number of available RPC channels.